This invention relates to mechanically-compensated zoom lenses particularly suited to extend the zooming range toward shorter focal lengths.
In providing a zoom lens of the type having a convergent front lens group for focusing stationary during zooming and two lens groups movable for zooming to effect a variation in focal length of the system with simultaneous effect of the image shift compensation, it has been proven that any improvements of the conventional design features are almost impossible in order to achieve further increases in the image angle at the wide-angle setting as well as in relative aperture and zoom ratio without causing unduly large increases in the overall dimensions and further without causing deterioration of image quality over the entire zooming range.
The conventional design features for increase in image angle at the wide-angle setting are:
(1) To increase the diverging power of the divergent lens group having a function of varying the focal length of the system (the so-called variator) which is positioned in rear of the focusing lens group (the convergent lens group rendered movable wholly or in part);
(2) To increase the axial distance between the positions which the diaphragm and the variator occupy at the wide-angle setting; and
(3) To decrease the overall focal length of the image-forming lens group (relay lens).
These three main methods are, however, associated with the following respective short-comings. The first-mentioned method tends to produce a large barrel-type (under-corrected) distortion at the wide-angle setting. This under-correction of distortion is usually compensated by bending the focusing lens to convexity in the rearward direction. By this operation, however, extreme under-correction of astigmatism will result and, when zoomed to the telephoto settings, extreme over-correction of spherical aberration is produced which is attributable to the variator since it has a strong diverging power. When the necessary compensation for such spherical aberration is concentrated either at the focusing lens or at the compensator, it is difficult to achieve good stability of correction of aberrations throughout the focusing range or the zooming range respectively.
The second method though capable of achieving somewhat good level of aberrational correction at the wide-angle settings calls for an increase in focal length of the focusing lens since the total length of the zooming section is increased. As a result, the focusing lens is required to perform an increased total axial movement which in turn calls for an increase in diameter of the front lens member, thereby the complete system is made quite heavy and bulky.
The third method, because of the decreased focal length of the relay lens, renders it difficult to secure a desired back focal length. In addition thereto, the angle of inclination of the extreme off-axis light bundle incident upon the front surface of the relay lens is increased with the resultant diameter of the front lens member of the system being increased. This leads to deterioration of distortion and astigmatism at the wide-angle positions.
As far as the conventional lens design features for the zoom type described above are concerned, therefore, it is very difficult to achieve simultaneous establishment of a great increase in image angle at the wide angle settings with great increases in relative aperture and zoom ratio, while nevertheless preserving good stability of correction of aberrations throughout the zooming range and the limitation of the overall dimensions to a minimum.
It is known to provide an alternate type of zoom lens including three movable lens groups for zooming, as, for example, disclosed in Japanese patent application laid open for inspection Sho 50-126441, which is assigned to the assignee of the present invention. The practical embodiments of the zoom lens disclosed therein are directed to the employment of a focusing lens group which has a negative refractive power. The three lens groups movable for zooming behind the focusing lens group have positive, negative and positive refractive powers respectively. During zooming, the front and middle movable groups are moved in the opposite directions to each other, while the rear movable group is moved independently of the preceding two movable groups. In Japanese patent application laid open for inspection Sho 50-149360, there is described a zoom lens comprising a positive movable lens group, a negative movable lens group, a negative movable lens group and a positive relay lens group. These three movable groups are simultaneously moved during zooming. The axial movement of the front movable group is regulated by a U-shape cam. The focusing of this zoom lens seems to be performed by moving the entire system.